Ophicalcite - Plate-tectonic evidence

by Ole Nielsen

Snaky words

Ophicalcite from Totalp near Davos
Ophicalcite from Totalp near Davos

A field trip is a great place to learn new words. Here is my newest one: "ophicalcite". Well calcite I knew of course - a white mineral, calcium carbonate, CaCO3 - but ophicalcite seemed to be something different. When I got home I consulted the Internet to get a better grip on this strange word.

Ophicalcite seems to be a sort of Chinese drug to stop heavy bleeding. If I got it right it is also good for the liver, and the taste is sour. My searches on "ophicalcite" further lead me through some pages on ornamental stones like green Swedish marble and Connamara marble, but didn’t make me much wiser, so when the word "ophicalcite" finally brought me to a couple of porno sites I shut down the computer and began to search through my paper texts.

Starting with looking for (the prefix) ophi in my Greek dictionary, I discovered that ophis means snake or serpent, and now things began to make sense. Serpent -> serpentine i.e. a "serpentine-calcite" rock or a calcite-bearing serpentinite. Serpentine has by the way not only got its name because of its banded or streaked texture, but also because it was used against snakebites. Maybe at this stage I should also admit that some old papers further disclosed that I in fact encountered the word ophicarbonates in January 1998.

Altered rocks - Metamorphism

Replacement of serpentinite (S) is documented by relics of pyroxene (iron magnesium calcium silicates (with SiO4 tetrahedra) with or without aluminium, sodium and lithium) floating in a red microsparic limestone matrix (L) [D. Bernoulli et al., 2001].
Replacement of serpentinite (S) is documented by relics of pyroxene (iron magnesium calcium silicates (with SiO4 tetrahedra) with or without aluminium, sodium and lithium) floating in a red microsparic limestone matrix (L) [D. Bernoulli et al., 2001].

A further look at a photo of this particular ophicalcite reconvinces me that we are dealing with a metamorphic rock. I have three textbooks on metamorphism, but none of them mention ophicalcite or ophicarbonate (they are more than 25 years old, I must admit). Enough though to tackle the problem with serpentinite, which is not only found in our ophicalcites but also as outcrops of serpentinite in the same area (Totalp).

Our story starts in the upper mantle. The mantle mainly consists of olivine (or peridotite which again predominantly consists of olivine). The general formula for olivine is R2SiO4 (where R in principle is magnesium and/or iron, but manganese and calcium may also be present). During hydrothermal metamorphism serpentinite forms by hydration of upper mantle peridotites.

Example of serpentinisation:

4Mg2SiO4 + 4H2O + 2CO2     heat   > Mg6Si4O10(OH)8 + 2MgCO3

or to say it with words, when olivine infiltrated with fluids (water and carbon dioxide) is heated it transforms into serpentine/serpentinite and magnesite (a carbonate). Water leads to hydration and carbon dioxide to carbonation (here into magnesite) and the heat might have been provided by intrusions of igneous rocks.

And now on to our ophicalcite. A breccia with clasts of serpentinite set in a calcitic matrix. Replacement of serpentinite by calcite is visible. Sediments (or basalts) must have been deposited directly on the serpentinised mantle crust (on ocean floor). This sort of tectonic breccia forms along faults and in particular transform faults, and we speak of cataclastic metamorphic rocks (cata- for catastrophic and —clastic for fragmentation). The photo is taken to the north of Davos in Switzerland in the Totalp, and the ophicalcites are part of an ophiolite complex.

Plate-tectonics

transform fault
transform fault

So we have a gabbro intruded serpentinised upper mantle overlain by serpentinite breccia and ophicalcite. Our ophicalcite is part of the evidence for uplifting, rifting and opening of an ocean in the Jurassic (203 Ma — 135 Ma). (Dating of some of these rocks yields an age of around 160 Ma). The temperature of the hydrothermal fluids (altered marine water/pore water) during the tectonically active ocean floor regime was probably between 100 and 170°C. The major part of the Totalp serpentinite is by the way a variable serpentinised lherzolite (type of peridotite), which is an indication for a slow-spreading rift (whereas harzburgite (peridotite with clinopyroxene – also called a depleted peridotite in connection with the partial melting in the mantle to produce oceanic crust) indicates rapid spreading (rapid or high-degree partial melting = high degree of depletion).

Opening of the Thetys
Opening of the Thetys

My favourite book on plate tectonics is The Evolving Continents by Brian F. Windley, (third edition from 1995). This book indeed describes a Jurassic ocean (the Piemont Ocean or Piemonte-Ligurian Ocean) between Europe and Adria, and even mentions ophicalcites by name as one of the pieces of evidence for transform faults (page 123). Unfortunately the word ophicalcite is not included in the index to this book. (If you have the book at hand I also recommend reading the bit on ophiolites starting on page 117). As this obviously brought me into Tethyan Plate Tectonics,  and on their reconstruction map of the Tethys 156 million years ago, which looks like the one next to the text.

This ocean is called (or is part of) the Alpine Tethys (in an older book of mine also called the Mesogea/the Mesogean Ocean — to have existed during the middle Jurassic).

To complicate things the Platta-Arosa zone was again metamorphosed under greenschist facies conditions during the main phase (Oligocene - Middle of Tertiary) of the Alpine Orogeny less than 65 million years ago.

A story

I have left out a lot of details and made cruel simplifications. I hope however to have shown that a rock has a history that it is willing to tell if you are patient and listen carefully. Rocks are like people: They have a family name. They have an age. They are products of their heritage and environment. They speak their own language or even dialect.

Attention

Let me finally show one of the dangers by oversimplification. I have only shown one metamorphic reaction with low-grade hydrothermal hydration and carbonation. If all metamorphic reactions included carbonation, metamorphic rocks would be a carbon sink in the carbon cycle and thus have influence on global climate. It is true that carbonate (incl. calcite) vein formation acts as a significant sink for CO2 in shallow crustal levels, but in fact most metamorphic reactions involve dehydration and decarbonation such as CaCO3 + SiO2 = CaSiO3 + CO2 (calcite + quartz to wollastonite + carbon dioxide) so it is rather the other way round and it has been postulated that metamorphic CO2 degassing from orogenic belts have been responsible for global greenhouse warming in the past (e.g. early Cenozoic). I don't know the ratio of metamorphic carbonation/decarbonation but calculations from the New England metamorphic belt indicated that 9 - 18% of the CO2 generated by metamorphism was lost to carbonate veins. So listen to the story, but be careful in your interpretation, we might get it totally wrong.

So what is ophicalcite?

In the Paris Opera House ten steps of green Swedish marble lead to a vestibule for ticket-sellers. A brecciated “ornamental marble” composed of dark green serpentine, with calcite veins running through it, and called verde antique was quarried at the classical locality Casambala, near Larissa, Thessalia, Greece. This sort of stone is found in many parts of the world, in particular within ophiolite complexes, and has been widely used as ornamental or facing stone. It is known as “verde antico”, “antique verd”, “verde alpi”, “lapis atracius”, “Connemara marble”, “Irish green marble”, “green Swedish Marble”, “serpentine marble” or just “ophite” (ophite by the way is known for giving the power to see visions and to hear voices, and I do in fact think it made me see the light! Although according to a French website “ophite affleure également à Lourdes mais ce n'est pas un miracle!” – ophite can also be found at Lourdes, but that is not a miracle – so again, I don’t know) or indeed “ophicalcite”. “Words, words, words” as Shakespeare said, but not “a rose is a rose is a rose”!

I'm still not sure what ophicalcites are. Ophicalcites are ...! / ophicalcites are not ... ! I am still wrestling with different definitions of "ophicalcite", which I can't make fit totally together, and different rocks that are all called ophicalcites, sometimes referred to as type I, type II (a.o. by M. Lemoine in 1987), etc., and sometimes far from similar. For instance it seems that American geologists normally see ophicalcites as something like “recrystalized limestone composed of calcite and serpentine and formed by dedolomitization of  siliceous dolomites”, whereas the ophicalcites I have referred to here are “tectono-sedimentary breccias”. In both cases we are however clearly talking about a combination of serpentine and calcite as the word itself indicates, and serpentine + calcite do indeed together have the necessary elements (Mg + Ca + C + O) to form dolomite.

References:

Birth of an Ocean, D. Bernoulli et al., Excursion A5 in IAS 2001 21st Meeting Davos Switzerland. Excursion Guides, p. 69.
"Ophicalcites": tectono-sedimentary breccias documenting intra-oceanic tectonics, D. Bernoulli and G. Manatschal in IAS 2001 21st Meeting Davos Switzerland. Abstracts & Programme, p. 40.
The Evolving Continents, Brian F. Windley, Third Edition, Wiley.
IGCP Project No. 369. Comparative Evolution of Peritethyan Rift Basins

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